Separate files for KisInputManager::Private

Summary:
This is a very simple separation of KisInputManager::Private into its own header and .cpp file. This makes changes to these classes easier to read. It will make the patch D270 more legible and D256 should be easily rebased.

I'll be on IRC to discuss the tablet patches.

Reviewers: dkazakov, rempt

Differential Revision: https://phabricator.kde.org/D324